The proliferation of 'clean' digital sound, as employed by CDs, NICAM stereo, DAT and hard disk recording systems used in studios, has gradually made many people intolerant of imperfect sound. Just as most people don't like to watch black and white films on a colour TV, so they tend not to want to listen to scratchy or hissy recordings.
This tendency is even more prevalent in those who make and record their own music. In the quest for perfect sound, many of us pay just as much attention to sonic quality and the cleanliness of our recordings as we do to the music itself. If you're among such sonic perfectionists, you may find it hard to listen to many of your noisier recordings, with all their flaws. Now, if you have a PC, a soundcard, and lots of hard disk space, it's possible to clean up your sound library -- or restore it, to use the jargon -- and with the price of CD writers having come down to about £500, putting the cleaned-up material onto CD is not out of the question either.
Sound restoration, and especially digital sound restoration, has so far been a very expensive business -- it's one almost as tough (and therefore costly) as searching for alien messages from outer space. This comparison is not as irrelevant as you might think; when radio telescopes listen in to electromagnetic waves from space, the signals are put into giant computers and analysed in order to extract the existence of any meaningful information from random noise. Digital audio restoration works on exactly the same principle: here too, the computer has to analyse the sound waveform and subtract the noise from the meaningful sound.
This a pretty tall order. In fact, there aren't that many systems in the world that can make a really good job of it. And the problem with systems that aren't good is that they can subtract meaningful sound content as well as noise, and make the resulting sound rather dull.
Among the best noise removal systems we know of is CEDAR, developed in Cambridge. Tracer Technologies' DART Pro (Digital Audio Restoration Technology) Windows PC software, under review here, doesn't match up to CEDAR, but it is pretty sophisticated. It's capable of removing both impulsive disturbances (clicks, pops and scratches) and wide-band noise (tape hiss and surface noise). And although it's not a real-time tool, the process doesn't take forever. Equally importantly, its price would get you only about 10 minutes of restored sound if you used CEDAR's bureau service!
DART Pro has an easy-to-use graphical interface, and a window synchronisation feature and various zoom options enable you to get around easily. We're also happy to report that the program includes useful on-line help, keyboard shortcuts, and [hooray -- Ed] a good manual.
Apart from restoration, DART Pro can be used to a certain extent as a sound editor, as it also allows you to record, edit and process sound files in a number of ways. The Record menu opens a separate window which allows you to record WAV files directly from within DART Pro, without the need for an external recording application. The sample rate and bit resolution of the resulting file can be set in this window. The Edit menu provides standard editing tools such as Cut, Copy and Paste, as well as others for marking points in a sound file and selecting local areas (Select Modified Area, Select Local Block). In addition to the standard Open, Save, and Delete options, the File menu also has a sound file format conversion option (RAW to WAV, WAV to RAW; no other formats are supported). The Play menu options are simpler, allowing you to play the entire sound file (Play All) or just a portion of it.
The Toolbox menu contains fairly general processing functions, such as Scale and Maximize, and those for muting, reversing, and fading a signal (in or out), as well as those for comparing and mixing two signals. The Compare option in the Toolbox menu is particularly useful -- it subtracts one waveform from another, and presents you with the result. If, after restoring a file, you subtract the restored waveform from the source file, the resulting file should, at best, simply contain noise, clicks and crackles. This helps you hear how successful your restoration session really is. The Split and Unite options, also found in the Toolbox, can be used to separate two channels of a stereo sound file, and to combine two mono tracks into a stereo sound file.
The Toolbox also contains other more complex processing functions, such as filtering (low-pass, high-pass, band-pass and notch types are available) and equalisation. The Equalise function brings up a 10-band graphic equaliser, while the Notch filter enables you, for example, to remove unwanted hums produced by mains current.
The Restore menu is where the most important functions in DART Pro are found. Digital sound restoration is a subjective process, and, as with using a tone control to reduce hiss, it's up to you to decide where to draw the line (though the program has default settings which are worth trying first). Depending on your needs, the system can attempt to restore an entire WAV file (using the Run All option) or just selected parts of it (using Run Window, or Run Block). The EasyRun option uses the default options and gives you a start, but it's worth selecting some settings yourself for the best results. Luckily, there are only three settings to try: the smoothing factor, the post-filtering factor, and the detection threshold. You also have to specify whether the sound has any vocals in it. See the 'If Noise Annoys' panel for more details about these processes. In some cases, the quality of a restored recording can be further improved by means of local 're-touching' -- just hit 'Process' again. If you overdo it, you can undo at every stage of the processing, even to the extent of undoing single changes to your source file. So if you're broadly happy with the results of the processing, but find that the program has dulled the first word of a vocal too much for your liking, you can undo just that change, and keep the rest of the processed file the same.
The DeNoise function, also found under the Restore menu, is a recent addition: it's a procedure for eliminating wide-band noise with certain known characteristics. Prior to using this function you have to 'noiseprint' an audio file, which involves running the NoisePrint noise identification function on a fragment of the recording that contains noise only -- for example, a short piece of crackly silence at the start of a recording. Alternatively, if no reference noise sample is available, you can try using a noiseprint extracted from another archive recording (preferably from the same source -- for example, from somewhere else on the same old record).
Finally, DeHiss, another new feature, is a wide-band noise reduction utility based on a standard noise model. You can use it when a noiseprint is not available at all. If the file's noise characteristics vary over time, the DeHiss function may yield better results than DeNoise -- because it is less selective, it is also less sensitive to changes in the noise profile.
We tested the program on half a dozen sound files, including one that featured a 1970s studio recording of some piano music that we had also restored using the CEDAR system. Although DART Pro was not as effective in removing hiss as CEDAR, it did comparatively well, not adversely affecting the sound or making it dull. We also had the opportunity to try the program with an original 78rpm disc recorded in 1952, which was badly scratched, as well as with recordings made on cassettes in the late '60s on domestic equipment. DART Pro does not remove the noise or clicks completely; it lowers their sound level, so that the real signal can come through (it is possible to remove noise completely, but some deterioration of the signal does occur). Some experimentation with the restoration parameters is also necessary, but it is certainly worth the time.
Processing time, on our 486/66MHz machine, proceeded at a ratio of about 10:1 for clicks and scratches, and about 5:1 for noise reduction -- in other words, it takes 10 minutes to remove the clicks and scratches from a one-minute, 44.1kHz, 16-bit stereo recording, and five minutes to de-hiss it. It also pays to select less severe settings for the restoration parameters and pass the audio through the program many times.
DART Pro is for users like you and us. It doesn't cost a lot, and as there are only three parameters to set, it's very easy to use. The disadvantage is that the results can range from excellent to unusable -- and you may have to experiment quite a bit before you get something good. Fortunately, when the results are good, they're a lot better than what you can achieve with the EQ on a mixing desk or other noise reduction systems associated with analogue tape decks; once we had got the hang of DART Pro, every recording we tested sounded better after it was processed than it did before. It's certainly worth the money if you have more than, say, half an hour of audio that needs cleaning up: it all depends on how important the material is to you. In all, this program is well designed, easy to use, and includes a wealth of effective features for its price. We definitely recommend it.
DART Pro's minimum system requirements are Windows 3.1 or later, 8Mb of RAM, and a large hard disk. A maths co-processor is also recommended for work on large audio files.
The heart of the DART Pro system is its adaptive renovation filter, which is a rather clever algorithm designed to isolate and reconstruct samples corrupted with clicks, pops, record scratches and other forms of impulsive noise, as well as removing wide-band noise (like tape hiss). The process consists of two stages. First, impulsive noise is removed, together with some wide-band noise. This process is known as smoothing, and is achieved with a special device known as a Kalman filter, which is used to reconstruct the signal. Secondly, a special adaptive post-filtering algorithm is used to further reduce the noise. This algorithm is designed for adaptive cancelling of wide-band noise, and is therefore particularly effective on tape hiss and surface noise. Smoothing is generally uniform, while post-filtering is selective: in other words, smoothing acts in the same way on all parts of the recording, while post-filtering suppresses most noise on silent parts and least noise on loud parts, where the noise is, of course, less audible. In this way, the sharpness in the louder parts of the original recording isn't lost.
The third user-definable setting is the detection threshold. This decides the sensitivity of the so-called outlier detector, a device used to localise impulsive disturbances. With the smoothing and post-filtering settings at zero, DART Pro will de-click and de-crackle the recording without attempting to remove the wide-band noise.
The algorithms used by DART Pro are pretty impressive, but they work much better on music than on speech (which is why the program asks you to specify whether your sound file has vocals in it). Speech is much harder to restore than music. Devoiced consonants ('K', 'S', 'T' and so forth) pose a problem, because it's hard for an algorithm to distinguish between them and, say, a crackle. Vowels pose an even bigger problem, because they have a pitch and are often very short, and pitch-related pulses can be easily confused with noise pulses caused by clicks or record scratches.
Cheap for what it offers.
Relatively fast, with fine control over every step of the restoration process.
Easy to use.
Don't expect miracles.
Can create dull sound if you are not careful.
Digital sound restoration isn't easy, and this program does very well for its price. The results depend on personal taste, but on most of our experiments it performed well.
£ DART Pro £299; DART (cut-down version lacking DeNoise and DeHiss options), £89; upgrade from DART to DART Pro for existing DART users, £49. Prices include VAT.
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